In false-twisting of a yarn of thermoplastic synthetic fibers, false twist is imparted to the yarn by a false-twisting means. As the false-twisting means, a false-twisting spindle unit is conventionally used because false twists of a number closely corresponding to a predetermined number of twists can be comparatively easily and stably obtained by the false-twisting spindle unit. In such a case, however, differences in the number of false twists may occur along the length of the yarn and/or between two or more of the spindles. In general, it has recently been a practice, in order to increase the processing speed of the false-twisting, that false twist is imparted to a thermoplastic synthetic yarn by using a friction type false-twisting unit or a fluid jet type false-twisting unit. Particularly in the case where the friction type or fluid type false-twisting unit is used, it is very difficult to detect and control the differences in number of twists along the yarn length or between two or more of the false-twisting means. This is because of the difficulty in ensuring that the yarn is twisted a particular number of twists corresponding to a predetermined number of twists and, thus, in determining the number of false twists being actually imparted to the yarn.
For the determining number of twists of a yarn being false twisted, some conventional methods have been utilized. One of the methods is a direct method wherein a yarn in the twisting zone of a false-twisting apparatus is taken out and the number of twists actually imparted to the yarn is measured by a twist counter. In this direct method, the actual number of twists can be determined approximately accurately, but this method has the drawbacks that the production operation has to be temporarily discontinued to take out the yarn by cutting the yarn being processed and that it is difficult to detect the minute variations in the number of twists along the length of the yarn.
A method of indirectly determining the number of twists of a yarn during a false-twisting operation has been proposed in Japanese Patent Publication No. 38-24769. In this method, the determination of the number of twists is effected by bringing a disc in contact with the yarn in the twising zone in a condition such that the axis of the yarn is parallel to the rotational axis of the disc, rotating the disc by frictional force, counting the number of rotations of the disc and estimating the number of twists from the number of rotations. This method has, however, the drawbacks that noticeable error easily appears due to slippage or differences in the quality or quantity of the applied oils, abnormal yarn rotation occurs when the disc is brought in contact with or removed from the yarn and, in addition, an unusual number of yarn rotations often occurs, due to the loss of rotational energy of the yarn, during the time the disc is in contact with the yarn. An improved method wherein the above-stated energy loss is intended to be decreased has been proposed in published Japanese Patent Application No. 49-13463. This method has also drawbacks similar to those mentioned above.
A further method has been proposed in Japanese Patent Publication No. 41-17942, wherein the determination of number of twists is effected by measuring the frequency of two-dimensional vibration in the direction perpendicular to the progressive direction of the yarn being processed. The drawbacks of this method are that the results are effected by various factors such as the tension, speed and thickness of the yarn and the length of the twisting zone and, thus, are inferior in accuracy and reproducibility. This method has a further drawback that a large scale apparatus is needed for the determination.
As hereinbefore described, in conventional false-twisting methods it has been very difficult to detect and control the differences in the number of false twists along the yarn length or between two or more of the false-twisting mens by determining the number of false twists being actually imparted to the yarn.